1. Field of the Invention
The present invention relates to an antenna-integral high frequency circuit which is miniaturized, has light weight and is superior easy to produce, and more specifically to an antenna-integral high frequency circuit having a superior high frequency characteristic.
2. Description of the Background Art
Recently, rapid and large-capacity personal communication using a high frequency wave, such as microwave and millimeter wave, has attracted attention with the recent improvement in the processing speed of information processors and in the resolution of image processing apparatuses. In such devices, a miniaturized, light-weight and high-performance microwave and millimeter wave transmitter/receiver can be implemented by not only simply integrating a microwave and millimeter wave circuit with an antenna but by also configuring the integrated structure to make use of the high frequency characteristics of utilized microwave and millimeter wave. Japanese Patent Laying-Open No. 8-56113 discloses an invention as one of such antenna-integral microwave and millimeter wave circuits.
FIG. 1 shows a structure of an antenna-integral microwave and millimeter wave circuit disclosed in Japanese Patent Laying-Open No. 8-56113. The antenna-integral microwave and millimeter wave circuit includes a grounding conductor film 102, a dielectric film 103 formed of silicon dioxide film, a plane antenna 104, a semiconductor substrate 105 with a detector circuit, a microstrip line 107 for feeding power to plane antenna 104, an output signal terminal 108 for outputting detected signals, a dielectric substrate 113, and a slot 116. Plane antenna 104 is formed on one surface of dielectric substrate 113. Grounding conductor film 102 including slot 116 is formed on the other surface of dielectric substrate 113, and dielectric film 103 is deposited thereon with grounding conductor film 102. Microstrip line 107 and output signal terminal 108 are formed on the other surface of dielectric film 103 and are connected to semiconductor substrate 105 including the detector circuit.
Microstrip line 107 and plane antenna 104 are coupled via slot 116 using an electromagnetic field and thus form an antenna-integral microwave circuit.
However, the invention disclosed in Japanese Patent Laying-Open No. 8-56113 has the following disadvantages.
(1) an antenna grounding conductor is not present near the antenna. Accordingly, the potential of plane antenna 104 is unstable, resulting in disturbance of the radiation pattern caused by a conductor, such as metal, located near the plane antenna, degradation in radiation efficiency, a cause of noise generation and the like.
(2) Even if grounding conductor film 102 serves as the plane antenna's 104 grounding conductor, a conductor grounding the microwave and millimeter wave circuit is still not connected to the antenna grounding conductor in the vicinity of the microwave and millimeter wave circuit. Accordingly, the antenna grounding conductor can not be a true ground with respect to the microwave and millimeter wave circuit 105 grounding conductor, the reason of which will be described later. Accordingly, the detector circuit formed at semiconductor substrate 105 is unable to achieve less noisy, stable operation, resulting in unstable operation, such as parasitic oscillation, and increased parasitic capacitance. At the plane antenna 104 side, electromotive force will occur in the conductor grounding the detector circuit formed at semiconductor substrate and in the plane antenna 104 grounding conductor and thus causes an unnecessary radiation pattern with high side lobe. Furthermore, variation of the input impedance of plane antenna 104 also occurs, resulting in a cause of degradation of the radiation efficiency of plane antenna 104 and a cause of variation of the resonance frequency of plane antenna 104.
(3) Among dielectric film 103 and dielectric substrate 113, dielectric film 103, connected to semiconductor substrate 105, is in the form of a thin film. Accordingly, for the frequency regions of microwave and millimeter wave, the conductor for microstrip line 107 must have a reduced width, which will in turn cause a large transmission loss. The strength of the entirety of the antenna-integral microwave and millimeter wave circuit depends almost only on dielectric substrate 113. However, the thickness of dielectric substrate 113 must be reduced to obtain the desired characteristic impedance (typically 50.OMEGA.) for the frequencies of microwaves and millimeter waves. Accordingly, it is difficult to handle, and a high production yield cannot be expected. Particularly, such a flip-chip connection of semiconductor substrate 105 as shown in FIG. 1 is typically made in high temperature environments and with a high current load, and a thin dielectric substrate 113 is thus not practical. The examples of dielectric film 103 include silicon dioxide, silicon nitride or polyimide. However, silicon dioxide and silicon nitride, which are resistant to heat, have low impact resistances and cracks are readily caused therein in flip-chip bonding. Polyimide is inferior in heat resistance and is thus difficult to use for flip-chip connection by thermocompression bonding, and such a technique must be employed by placing resin between semiconductor substrate 105 and dielectric film 103 to connect them through adhesion. Accordingly, the insufficient bonding in a conductor transmitting signals between semiconductor substrate 105 and output signal terminal 108 or microstrip line 107, which is not particularly disadvantageous for direct current or low frequency, readily causes reflection in high frequency. The use of resin also increases the apparent dielectric constant of dielectric film 103 and thus results in more stringent limitations on circuit design. Furthermore, resin can inherently be significantly disadvantageous in reliability. Polyimide is relatively soft and plastically deformed and is thus difficult to repair.
(4) The deposition of only a single layer (the plane antenna 104 layer) on one surface and three layers (the grounding conductor film 102, dielectric film 103 and microstrip line 107 layers) on the other surface of dielectric substrate 113 requires a increased number of manufacturing steps. After each step, the difference in internal stress is increased between the layer on one surface and a layer on the other surface, causing disadvantages such as bowing. It is thus difficult to obtain a good high frequency characteristic.
(5) Since not only semiconductor substrate 105 but dielectric substrate 113 (including grounding conductor film 102, dielectric film 103, microstrip line 107, output signal terminal 108, dielectric substrate 113, plane antenna 104 and slot 116) mounted with semiconductor substrate 105 are manufactured by a semiconductor process, caputalization and running costs are higher and the mass productivity is lower than those of e.g., thick-film print processes. Although a semiconductor process is superior in the precision in radiation pattern, thick-film print process also achieves a practically sufficient level of precision in radiation pattern. While the precision in flip-chip mounting rather depends on the precision of the flip-chip bonder, the recent bonding equipment has achieved the required precision level.